extmk.rb: fix mixing mkmf.log
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@37879 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
extmk.rb: fix mixing mkmf.log
failure messages are not mixed.
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@37879 b2dd03c8-39d4-4d8f-98ff-823fe69b080e